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Diverse Northeast Monsoon On Account Of El Nino And La Nina

October 27, 2023 2:03 PM |

Northeast monsoon has made a poor start across South Peninsula. Although, the arrival of monsoon was announced on 21stOctober which nearly coincided with its normal date of 20thOctober, but the typical feel of change of season is yet to witness. The week gone by from 19th – 25th  October was the driest period for entire South Peninsula, outside state of Kerala.  South Peninsula recorded 6.3mm rainfall against the normal of 33.1mm, a shortfall of 81%. There are rains likely towards the fag end of the month but not adequate enough to better the rainfall stats of October.

Northeast monsoon is the chief rainy season for southern meteorological sub divisions but more so for the state of Tamil Nadu. The state receives 48% of its annual rainfall during the three month old season, from October to December. Long period average( LPA) for the entire South Peninsula is 334.13mm during northeast monsoon. This is based on 50 years data  from 1971 to 2020.  Out of five sub divisions covered by northeast monsoon, namely Tamil Nadu, Kerala, Coastal Andhra Pradesh, Rayalaseema, South Interior Karnataka, Kerala is the rainiest with 491.6mm rainfall, closely followed by Tamil Nadu with 449.6mm.

Northeast Monsoon shows high degree of variability and the coefficient of variation is 27%. As such, the range of normal rainfall also widens. As compared to the normal range of 96% to 104% rainfall of LPA of southwest monsoon season, northeast monsoon has a normal range of 88% to 112%. In its season’s long range forecast, the National Weather Service had predicted normal rainfall for northeast monsoon 2023. It also forecast a normal rainfall of 85% -115% rainfall of LPA for the month of October 2023. This target is likely to fall short by a big margin. As of today, the month is deficit by 28% rainfall which may marginally improve by month end.

Northeast monsoon is influenced by global climate parameters such as ENSO, IOD, MJO.  El Nino,  +VE IOD and MJO in Phase 2-4 with amplitude >1 are generally associated with better northeast monsoon rains.  Also, La Nina and -VE IOD mostly corrupt the northeast monsoon. This was largely vindicated by two diverse northeast monsoon of 2015 and 2016. Year 2015 was a strong El Nino year with positive IOD, where as 2016 proceeded under the shadow of La Nina, coupled with  negative IOD.  Monsoon 2015 ended with a surplus of  huge 52% rainfall with unprecedented floods over north coastal districts of Chennai, Kanchipuram and Thiruvallur.  Northeast monsoon 2016 finished with disastrous shortfall of 62% rainfall, one of the  record highest in the recent past.

The current season has made a dismal start with October likely to go as driest on record, since last 5 years.  This can be attributed to the impact of El Nino on the retreating southwest monsoon. Recovery is likely under  the influence of El Nino and positive IOD in the core monsoon months of November and December. MJO, a transiting parameter may also become favourable around middle of November.

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